1. Field of the Invention
The present invention relates to an apparatus such as an image-shake preventing apparatus for preventing the shake of an image, and to a control method therefor.
2. Description of Related Art
It is known that, in an image pickup apparatus such as a small-sized video camera, a picked-up object image shakes due to the vibration of the image pickup apparatus, so that a video image intolerable to view might be outputted or recorded.
In particular, in such a kind of image pickup apparatus, it has become general these days that a zoom lens capable of continuously varying the focal length without changing the image point position is mounted in the image pickup apparatus, and, in recent years, an image pickup apparatus having a high zoom magnification ratio of ten or more times has widely come into the market. However, such an image pickup apparatus has a drawback that, when an object image is picked up with the setting of the telephoto side, which is set for the larger zoom magnification, a conspicuous shaking of the object image would occur.
Therefore, as measures to solve the above drawback, an image pickup apparatus having mounted therein an image pickup optical system having image-shake correcting means has been developed so far and has already been put on the market.
FIG. 18 is a conceptual diagram schematically showing the above-mentioned image pickup optical system, which is denoted by reference numeral 200. In the image pickup optical system 200, there are disposed, in order, a fixed lens 201 securely fixed to a lens barrel (not shown), a variator lens 202 arranged to move in the horizontal direction on an optical axis "c" as indicated by an arrow "a", a shift lens 203 arranged to move two-dimensionally within a plane perpendicular to the optical axis "c" (in the direction indicated by an arrow "b", a focusing lens 204 having the focus adjusting function and the function of correcting the movement of a focal plane resulting from the movement of the variator lens 202, and an image sensor 205 on which to form an object image. Further, in the respective predetermined positions adjacent to the shift lens 203, there are disposed an actuator 206 arranged to drive the shift lens 203 and a position detecting sensor 207 arranged to detect the position of the shift lens 203.
In the image pickup apparatus 200, even if, as shown in FIG. 19(a), the optical axis "c" deviates from a central axis "c'" of the image pickup optical system 200 due to the vibration thereof as much as a deviation angle e, it is possible to make the optical axis "c" and the central axis "c'" of the image pickup optical system 200 geometrically coincident with each other on the downstream side of the shift lens 203, by driving the actuator 206 to move the shift lens 203 as indicated by an imaginary line in FIG. 19(b). Accordingly, the above-mentioned deviation angle .theta. is corrected by an optical processing, so that the object image is formed on the image sensor 205 as a light flux having no shaking.
FIG. 20 is a block diagram showing the arrangement of a conventional image pickup apparatus which corrects an image shake by means of the image pickup optical system 200.
In the image pickup optical system shown in FIG. 20, when a power supply switch 208 is turned on, a mode microcomputer 209 notifies a main microcomputer 210 of the turning-on of the power supply switch 208. Then, having determined that the power supply has been turned on, the main microcomputer 210 starts its control operation.
Subsequently, a vibration signal forming circuit 211, which has detected the vibration of the body of the image pickup apparatus, forms a vibration signal and supplies the vibration signal to a vibration correcting circuit 212. In the vibration correcting circuit 212, the analog vibration signal is converted into a digital vibration signal by an A/D converter 213, and, then, a predetermined low-frequency component is removed from the digital vibration signal by a high-pass filter (HPF) 214. After that, the phase and gain of an output signal of the HPF 214 are corrected by a phase/gain correcting circuit and an output signal of the phase/gain correcting circuit 215 is integrated by an integration circuit 216 to calculate and output a correction target value.
The correction target value outputted from the vibration correcting circuit 212 is converted into an analog value by a D/A converter 217 and is then supplied to an adder 218. At the adder 218, the analog correction target value is added to a feedback signal supplied from the position detecting sensor 207 through an amplifier 219. Then, an output signal of the adder 218 is supplied to a driving circuit 220. The driving circuit 220 issues a driving signal to the actuator 206 to drive the shift lens 203.
When the shift lens 203 is driven by the actuator 206, as described above, the deviation angle e is optically corrected, so that the object image is formed on the image sensor 205 as a light flux having no shaking.
Further, an electric signal obtained through the photo-electric conversion by the image sensor 205 is supplied to a video signal processing circuit 222 via a camera signal processing circuit 221. Then, a video signal produced by the video signal processing circuit 222 is outputted to an output terminal 223 so as to be converted into a visible video image on the display screen, and, at the same time, is recorded, as video information in the form of an RF signal, on a recording medium such as a magnetic tape by a recorder 224.
Incidentally in the above-mentioned image pickup apparatus, the actuator 206 for driving the shift lens 203 is composed of a voice coil motor.
More specifically, the voice coil motor is disposed in a predetermined position adjacent to the shift lens 203. By causing current to flow to the voice coil motor to generate an electromagnetic force, the shift lens 203 is made to float, and by varying the electromagnetic force according to an output of the adder 218, the shift lens 203 is made to two-dimensionally move within a plane perpendicular to the optical axis "c" in the vertical direction (in the pitching direction) and in the horizontal direction (in the yawing direction).
However, since, in the conventional image pickup apparatus, as described above, the actuator 206 is composed of a voice coil motor, the shift lens 203 is held in a floating state by the voice coil motor when the voice coil motor is a conductive state with the power supply switch 208 turned on, but, when the power supply is turned off, the holding force for the shift lens 203 by the voice coil motor is canceled, so that the shift lens 203 drops due to its own weight. As a result, a lens holding frame which holds the shift lens 203 collides with an inner wall of the lens barrel to generate a collision sound, which is offensive to the ear.
Further, since the optical axis "c" decenters due to the movement of the shift lens 203, for example, if the power supply is turned off during the process of an image pickup operation of the image pickup apparatus, there is a possibility that a video image having an unnatural motion is outputted or recorded on the recording medium.